#include <bits/stdc++.h>
using namespace std;

#define int int64_t
#define endl "\n"
using PII = pair<int, int>;
using TII = tuple<int, int, int>;
#define FOR(i, a, b) for (int i = (int)(a); i <= (int)(b); i++)
#define FOR2(i, a, b, c) for (int i = (int)(a); i <= (int)(b); i += c)
#define FORD(i, b, a) for (int i = (int)(a); i >= (int)(b); i--)
#define FORD2(i, b, a, c) for (int i = (int)(a); i >= (int)(b); i -= c)
#define BSI basic_string<int>
#define BSPI basic_string<PII>
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
#define VI vector<int>
#define VII vector<vector<int>>
#define VPII vector<PII>
#define lowbit(x) ((x)&(-x))
#define RE return;
#define RET return true;
#define REF return false;
#define Yes cout << "Yes" << endl;
#define YES cout << "YES" << endl;
#define No cout << "No" << endl;
#define NO cout << "NO" << endl;
#define pb push_back
#define fi first
#define se second
#define sz size()
constexpr int N = 2e5 + 10;
constexpr int mod = 998244353;

int __FAST_IO__ = [](){
    ios::sync_with_stdio(0), cin.tie(0);
    cout.tie(0);
    cout << fixed << setprecision(12);
    return 0;
}();

string s;
int n;

void solve() {
    cin>>s;
    n=s.sz;

    map<char,int>mp;

    for(auto x:s)mp[x]++;

    if(n&1){
        int cnt=0;
        for(auto [x,y]:mp){
            if(y&1){
                cnt++;
            }
        }
        if(cnt>=2){
            cout<<"NO SOLUTION";
            RE;
        }

        int l=0,r=n-1;

        for(auto& [x,y]:mp){
            if(!(y&1)){
                while(y){
                    s[l++]=x;
                    s[r--]=x;
                    y-=2;
                }
            }else{
                while(y!=1){
                    s[l++]=x;
                    s[r--]=x;
                    y-=2;
                }
                s[(n)/2]=x;
            }
        }

    }else{
        for(auto [x,y]:mp){
            if(y&1){
                cout<<"NO SOLUTION";
                RE;
            }
        }
        int l=0,r=n-1;
        for(auto& [x,y]:mp){
            while(y){
                s[l++]=x;
                s[r--]=x;
                y-=2;
            }
        }
    }

    cout<<s;

}

signed main() {
    int Task = 1;
    for (; Task; Task--) {
        solve();
    }
    return 0;
}